Plate-Type Heat Exchanger With Anodic Corrosion Protection

ABSTRACT

To protect a plate-type heat exchanger ( 1 ) against corrosion due to the attack of sulfuric acid, it is proposed in accordance with the invention that the region through which flows sulfuric acid has at least one metal cathode ( 16, 17 ) and one reference electrode ( 27 ), that at least half the metal plates ( 7 ) have an electric contact ( 23 ) which is connected with the anode ( 21 ) of an electric d.c. voltage source of variable electric voltage, that the metal cathode ( 16, 17 ) likewise is electrically connected with the d.c. voltage source, and that the d.c. voltage source belongs to a potentiostat ( 20 ) which is electrically connected with the reference electrode ( 27 ).

[0001] This invention relates to a plate-type heat exchanger comprisingnumerous parallel metal plates, between which there are alternatelyformed permeable cold chambers for cooling liquid and permeable hotchambers for the hot liquid to be cooled, comprising a housingsurrounding the plates, which housing has supply lines and dischargelines for the liquids, comprising a first distribution passage forsupplying the hot liquid to the hot chambers, comprising a seconddistribution passage for supplying the cooling liquid to the coldchambers, and comprising a first collecting passage for discharging thehot, cooled liquid and a second collecting passage for discharging thecooling liquid.

[0002] It is the object underlying the invention to protect suchplate-type heat exchanger against corrosion due to the attack ofsulfuric acid. In accordance with the invention, this is achieved inthat the metal plates and the housing are designed for the passage ofsulfuric acid as hot liquid and for the passage of water as coolingliquid, that the region through which flows sulfuric acid has at leastone metal cathode and one reference electrode, that at least half themetal plates have an electric contact which is connected with the anodeof a d.c. voltage source of variable electric voltage, that the metalcathode likewise is electrically connected with the d.c. voltage source,and that the d.c. voltage source belongs to a potentiostat which iselectrically connected with the reference electrode.

[0003] Expediently, a metal cathode is disposed in the firstdistribution passage and/or in the first collecting passage, where itgets in direct contact with the sulfuric acid. In particular in the caseof large-surface metal plates it may be expedient to pass a metalcathode through a plurality of hot chambers, which metal cathode issealed against the metal plates and electrically insulated. There isthus obtained contact with the sulfuric acid flowing in the chambers.

[0004] The anodically protected metal plates, which are equipped withassociated electric contacts, can for instance also have 2 to 5 electriccontacts per plate, when the plates should rather uniformly be protectedagainst corrosion in all regions. The housing can also have one or moreelectric contacts which are connected with the variable d.c. voltagesource, in order to achieve an anodic protection. Expediently, all metalplates will be protected anodically. By means of the anodic protection,a metal oxide layer is produced on the side against which flows thesulfuric acid, which metal oxide layer prevents the corrosion attack.

[0005] The plate-type heat exchanger protected against corrosion inaccordance with the invention can be used for instance in plants forproducing sulfuric acid, in which sulfuric acid with a H₂SO₄ content inthe range from 90 to 100 wt-% and temperatures in the range from 140° C.to the boiling point must be cooled by indirect heat exchange. Theplates may be made of alloyed steel which apart from iron in particularhas the alloying components chromium, nickel and molybdenum.

[0006] Embodiments of the plate-type heat exchanger will be explainedwith reference to the drawing, in which:

[0007]FIG. 1 shows a section through the plate-type heat exchanger in aschematic representation,

[0008]FIG. 2 shows a variant of the arrangement of a metal cathode, and

[0009]FIG. 3 shows a reference electrode in a longitudinal section in aschematic representation.

[0010] The plate-type heat exchanger (1) of FIG. 1 has a housing (2)comprising a supply line (3) and a discharge line (4) for the hotsulfuric acid to be cooled as well as a supply line (5) and a dischargeline (6) for the cooling water. In the housing (2), parallel metalplates (7) are provided, between which there are disposed hot chambers(8) for the passage of sulfuric acid to be cooled and cold chambers (8)for the passage of cooling water. The acid enters through the supplyline (3) and first of all gets into a first distribution passage (10),from where it flows through the hot chambers (8) to a first collectingline (11) and leaves the exchanger (1) through the discharge line (4).The cooling water enters the exchanger (1) through the supply line (5)and is supplied by the second distribution passage (13) to the coldchambers (9), reaches the second collecting passage (4) and then thedischarge line (6).

[0011] A first metal cathode (16) is provided in the first distributionpassage (10), and the insulated metal cathode extends through thehousing (2). Analogously, a second metal cathode (17) is provided in thefirst collecting passage (11). During the operation of the exchanger(1), both cathodes (16) and (17) are in contact with the sulfuric acid,whose corrosion attack must be stopped. In the vicinity of the contactwith the sulfuric acid, the cathodes are made of stainless steel, forinstance, which has a high resistance to hydrogen embrittlement, andoutside the housing (2) they are connected with the negative poles (19)and (19 a) of a potentiostat (20) by electric lines (18) or (18 a). In amanner known per se, the potentiostat has a variable d.c. voltagesource, whose positive pole (21) is connected with the electric contacts(23) of the metal plates (7) to be protected via the electric line (22).The housing (2) also has an electric contact (23 a), in order to obtainan anodic corrosion protection. In contrast to the drawing, each of theplates (7) to be protected can have a plurality of electric contacts(23) connected with the positive pole (21), e.g. 2 to 5 electriccontacts per plate. Mostly, it is expedient to dispose the contacts (23)at the edges of the plates, in order to achieve a constructionallysimple realization of the anodic protection.

[0012] The potentiostat (20) has a connection (25) for an electric line(26) which leads to a reference electrode (27). This reference electrode(27) provides the measurement basis for the potentiostat (20), and in amanner known per se it may constitute e.g. a calomel electrode, aHg/Hg₂SO₄ electrode, or a cadmium bar as shown in FIG. 3. Surrounded byan electric insulation (31), the cadmium bar (30) is provided in ahousing (32) which has a diaphragm (33). This diaphragm is permeable forthe sulfuric acid, so that the cadmium bar (30) is constantly immersedin the sulfuric acid flowing past the same. The potential developedthereby is supplied to the potentiostat (20) by the electric line (26).

[0013] In the case of large metal plates the anodic protection in themiddle region of the plate surface can possibly be smaller than in thevicinity of the plate edge. To provide sufficient anode current for thedesired corrosion protection in this case as well, it may be expedientto pass a metal cathode through the middle region of the plates (7), asis schematically represented by means of FIG. 2. The cathode constitutesa metal bar (16 a), and the sealed and electrically insulated cathodeextends through the plates (7). In this way, the anode current necessaryin the plate region susceptible to corrosion can precisely bedetermined.

1. A plate-type heat exchanger comprising numerous parallel metalplates, between which there are alternatively formed permeable coldchambers for cooling liquid and permeable hot chambers for the hotliquid to be cooled, a housing surrounding the plates, which housing hassupply lines and discharge lines for the liquids, a first distributionpassage for supplying the hot liquid to the hot chambers, a seconddistribution passage for supplying the cooling liquid to the coldchambers, and a first collecting passage for discharging the hot, cooledliquid and a second collecting passage for discharging the coolingliquid, the metal plates and the housing being designed for the passageof sulfuric acid as hot liquid and for the passage of water as coolingliquid, the region through which flows sulfuric acid having at least onemetal cathode and one reference electrode, at least half the metalplates having a plurality of electric contacts connected with the anodeof a d.c. voltage source of variable electric voltage, the metal cathodelikewise being electrically connected with the d.c. voltage source, andthe d.c. voltage source belonging to a potentiostat which iselectrically connected with the reference electrode.
 2. The plate-typeheat exchanger as claimed in claim 1, wherein a metal cathode isdisposed in the first distribution passage.
 3. The plate-type heatexchanger as claimed in claim 1, wherein a metal cathode is disposed inthe first collecting passage.
 4. The plate-type heat exchanger asclaimed in claim 1, wherein a metal cathode extends through a pluralityof hot chambers, which metal cathode is sealed against the metal platesand electrically insulated.
 5. The plate-type heat exchanger as claimedin claim 1, wherein a plurality of metal plates have 2 to 5 electriccontacts per plate.